Method for controlling evaporation temperature in an air conditioning system

ABSTRACT

A method for controlling evaporation temperature in an air conditioning system, especially an air conditioning system in a motor vehicle, comprising a latent cold accumulator which can be cooled by an evaporator ( 1 ). The evaporation temperature of a coolant is adjusted inside the evaporator ( 1 ) according to requirements to a value lying between a minimum temperature (T_min) and a maximum temperature(T max) lying below the phase transition temperature of the latent medium.

The invention relates to a method for controlling evaporationtemperature in an air conditioning system, in particular a vehicle airconditioning system.

A method for controlling the evaporator temperature in a vehicle airconditioning system is known, for example, from DE 199 20 093 C1. Toavoid unnecessary energy consumption, the evaporator temperature is tobe set to a temperature level which takes account of both comfort andsafety aspects. In this context, both the atmospheric humidity and therequired cooling capacity are taken into consideration.

A further evaporator temperature control, which is dependent on theoutside dewpoint, for a motor vehicle air conditioning system is known,for example, from DE 197 28 578 C2. In this case, the evaporatortemperature is dependent on the difference between air temperature anddewpoint.

However, the methods listed have the drawback that there is norefrigeration accumulator, and therefore the respective air conditioningsystem cannot be used when the vehicle is stationary and therefore thecompressor of the air conditioning system is not operating.

A vehicle air conditioning system having a refrigeration accumulator isknown, for example, from DE 101 56 944 A1. In this case, a refrigerantevaporator, for example a flat tube evaporator, has a number ofaccumulators filled with a refrigeration accumulator medium.Refrigeration accumulator media mentioned include decanol andtetradecane. When the evaporator is operating, the refrigerationaccumulator medium is cooled to a temperature below the melting point ofthe refrigeration accumulator medium. This creates a latent accumulatorwhich allows the cooling to be temporarily maintained when the vehicleand the refrigeration circuit are temporary stationary.

The invention is based on the object of providing a method with which anair conditioning system, in particular a vehicle air conditioningsystem, having a latent refrigeration accumulator can be operatedparticularly economically.

This object is achieved according to the invention by a method havingthe features of claim 1. This provides the possibility of controllingevaporation temperature in an air conditioning system that includes alatent refrigeration accumulator. The evaporation temperature of therefrigerant of the air conditioning system is in this case varied as afunction of demand between a minimum temperature and a maximumtemperature, which is below a phase change temperature of the latentmedium. The minimum temperature is preferably selected in such a mannerthat there is no possibility of the evaporator icing up. The meltingpoint of the latent medium contained in the latent refrigerationaccumulator is preferably slightly above 0° C. Decanol (melting point 7°C.) and tetradecane (melting 6° C.), as well as substance mixturescontaining at least one of these substances, are particularly suitablefor use as the latent medium. The maximum temperature of the evaporatoris preferably set to a temperature slightly below the melting point ofthe latent medium. Therefore, the evaporation temperature is always in arange in which both full usability of the latent medium is ensured andicing of the evaporator is avoided.

The method is suitable in particular for air conditioning systems invehicles which include what is. known as the idle-stop operating mode.In this case, the vehicle engine is automatically switched off when thevehicle is stationary for a short period of time, for example when ithas stopped at traffic lights. This also deactivates the compressor ofthe air conditioning system. The melting point of the latent mediumshould on the one hand be high enough to enable the evaporator withevaporator temperature control, also known as ETC, to operate in thewidest possible range. On the other hand, the melting point of thelatent medium should be low enough to allow a sufficient cooling actionto be maintained for a limited period of time when the refrigerationcircuit is stationary. The refrigeration storage media mentioned above,namely decanol and tetradecane, take sufficient account of thesecompleting conditions.

The advantage of the invention is in particular that operation of anevaporator of a motor vehicle air conditioning system with a controlledevaporation temperature, the upper limit of which is formed by themelting point of a latent medium, allows both particularly economicaloperation of the air conditioning system and the cooling operation to betemporarily maintained when the refrigeration circuit is stationary.

The following text provides a more detailed explanation of an exemplaryembodiment of the invention with reference to a drawing, in which:

FIGS. 1 a and 1 b show an evaporator of a vehicle air conditioningsystem that is suitable for carrying out the method according to theinvention,

FIGS. 2 a and 2 b shows diagrammatic cross-sectional illustrations of ineach case a device for cooling and reheating air in a vehicle airconditioning system, and

FIG. 3 shows a diagram illustrating various methods for evaporatortemperature control in a vehicle air conditioning system.

Corresponding parts and parameters are provided with the same referencedesignations throughout all the figures.

FIGS. 1 a and 1 b respectively show a perspective illustration and anexcerpt of an exploded illustration of an evaporator 1 which is suitablefor carrying out the method according to the invention and is designedas an accumulator evaporator. The basic structure of an accumulatorevaporator of this type is known, for example, from DE 101 56 944 A1.The evaporator 1 includes, as part of a vehicle air conditioning system(not shown in more detail) a number of flat tubes 2, as well asrefrigeration accumulator 3 and corrugated fins 4, which bear againstone another in the above order, in such a manner that air which is to becooled can flow through the evaporator 1 through the corrugated fins 4.Evaporating refrigerant, for example R 134 a, flows through the flattubes 2. The refrigeration accumulators 3, which are in each casecoupled to a flat tube 2, are designed as latent refrigerantaccumulators and are filled with a latent medium, for example decanol ortetradecane, as heat accumulator medium. The latent media mentioned havethe advantage of not being subject to any increase in volume when theyfreeze. As an alternative to the flat latent refrigeration accumulators3 provided in the exemplary embodiment shown in FIGS. 1 a, 1 b, it isalso possible to provide any other desired refrigeration accumulatorsfilled with a latent medium, for example in the form of capsulesarranged, in particular clamped, between the tubes 2 of the evaporator1.

The evaporation temperature in the accumulator evaporator 1 iscontrolled in such a manner that the latent medium always remainsfrozen, and therefore its enthalpy of fusion can be utilized if therefrigeration circuit is temporarily stationary, in particular inidle-stop mode. At the same time, the lower limit of the evaporationtemperature is just above 0° C., in order to prevent the evaporator fromicing up. A compressor whose displacement volume can be varied is usedin a manner known per se, for example as proposed in DE 199 20 093 C1,to control the evaporation temperature and to adapt the power of the airconditioning system.

FIGS. 2 a and 2 b illustrate various methods for cooling and partiallyreheating the air flowing through the evaporator 1. The air which flowsthrough the evaporator in the direction of flow 5, indicated by arrows,is intended to control the temperature of three ventilation areas R1,R2, R3 in the passenger compartment of the vehicle. In the exemplaryembodiment shown in FIG. 2 a, all of the air flowing through theevaporator 1 is passed to a radiator 6 a, the heating power of which canbe set by means of a valve 7. The valve 7 controls the flow of liquid,in particular the flow of water, through the radiator 6 a. This liquidor water control sets the temperature of the air flowing into theinterior of the vehicle to a desired value suitable for vehicle airconditioning.

In the exemplary embodiment shown in FIG. 2 b, a radiator 6 b whereofthe heating power cannot be controlled is connected downstream of theevaporator 1. In this case, the air outlet temperature is controlled bymeans of a mixing flap 8, which is arranged between the evaporator 1 andthe radiator 6 b and allows any desired part-stream of the air flowingthe evaporator 1 to be heated. In this case, the air conditioning systemis air-controlled.

FIG. 3 shows a diagram presenting various methods for controlling thetemperature in a motor vehicle air conditioning system. The figure alsoshows an outline illustration of a vehicle. The temperature profile ofthe air flowing into the vehicle is illustrated below this outline. Theair temperature T is in this case recorded as a function of a flowdistance s for a first conventional operating method v1, a secondconventional operating method V2 and the operating method according tothe invention V3. In all the methods V1, V2, V3, the ambient temperatureis, for example, 24° C., the interior temperature in the vehicle is 20°C. and the air outlet temperature from the air conditioning system is12° C. The air which is passed into the interior of the vehicle from theoutside first of all flows through the evaporator 1 and then a radiator6, as illustrated diagrammatically in FIGS. 2 a and b.

The evaporation temperature cannot be controlled using the firstconventional method V1. The evaporator 1 is always operated at maximumpower. The air flowing through evaporator 1 is in this case cooled toapprox. 0° C. Then, the air is heated again in the radiator 6 to 12° C.This first method V1 entails an unnecessarily high energy consumption.

According to the second conventional method V2, known as the ETC method,the evaporation temperature can be controlled between approx. 0° C. andapprox. 12° C. In the exemplary embodiment illustrated, the air in theevaporator 1 is only cooled to 12° C. In this case, the radiator 6connected downstream from the evaporator 1 does not function. Thissecond method V2 is distinguished by a relatively low energyconsumption. However, it is not possible by means of the second methodV2 to freeze a latent medium, for example decanol or tetradecane, underall operating conditions. Therefore, the second method V2 is notsuitable for a vehicle with an idle-stop operating mode.

According to the third method V3, in accordance with the invention, thetemperature range within which the evaporation temperature can becontrolled is restricted to the range between the minimum temperatureT_(min), which is fixed at approx. 0° C., and the maximum temperatureT_(max), which is set to 6° C. In the exemplary embodiment illustrated,the evaporator 1 cools the air flowing through it to 6° C., so that thelatent medium in the latent refrigeration accumulator 3 just remainsfrozen. As an alternative to a latent refrigeration accumulator 3integrated in the evaporator 1, it is also possible to use arefrigeration accumulator which is arranged between the evaporator 1 andthe radiator 6 and is “charged”, i.e. cooled, by the air stream cooledin the evaporator 1. The air which has been cooled to 6° C. is thenreheated to 12° C. in the radiator 6. This allows economical operationof the air conditioning system, with the latent refrigerationaccumulator 3 remaining permanently charged when the refrigerationcompressor is running.

List of Designations

-   1 Evaporator-   2 Flat tube-   3 Latent refrigeration accumulator-   4 Corrugated fins-   5 Direction of flow-   6 a, 6 b Radiator-   7 Valve-   8 Mixing flap-   R1 to R3 Ventilation area-   s Flow-   T Temperature-   T_(min) Minimum temperature-   T_(max) Maximum temperature-   V1 to 3 Method

1. A method for controlling evaporation temperature in an airconditioning system, in particular a vehicle air conditioning system,having a latent refrigeration accumulator (3) which can be cooled by anevaporator (1), with the evaporation temperature of a refrigerant in theevaporator (1) being set as a function of demand to a value between aminimum temperature (T_(min)) and a maximum temperature (T_(max)) whichis below a phase change temperature of the latent medium.
 2. The methodas claimed in claim 1, characterized in that the latent refrigerationaccumulator (3) contains decanol as latent medium.
 3. The method asclaimed in claim 1, characterized in that the latent refrigerationaccumulator (3) contains tetradecane as latent medium.
 4. A vehicle airconditioning system to be operated by the method as claimed in claim 1.